Compounds of formula (I), wherein A is S or O; x is 0 or 1; a is SR.sub.10 or N(R.sub.11)(R.sub.12); R.sub.1 and R.sub.2 are C.sub.1 C.sub.24alkyl, OR.sub.10, CF.sub.3 or halogen; R.sub.3, R.sub.4 and R.sub.5 are hydrogen, C.sub.1 C.sub.24alkyl, OR.sub.10 or halogen; or two of the radicals R.sub.11 R.sub.21 R.sub.31 R.sub.4 and/or R.sub.9 together form C.sub.1 C.sub.2oalkylene uninterrupted or interrupted by O, S or NR.sub.13; R.sub.6, R.sub.11 R.sub.8 and R.sub.9 are hydrogen, C.sub.1 C.sub.24alkyl; C.sub.2 C.sub.24alkyl which is interrupted one or more times by non-consecutive O atoms and unsubstituted or substituted by OH and/or SH, or R.sub.6, R.sub.7, R.sub.8 and Rg are OR.sub.10; halogen; or phenyl unsubstituted or substituted one or more times by C-Calkyl; R.sub.10, R.sub.11 and R.sub.12 are e.g. hydrogen; X is e.g. (II), and R.sub.2; have one of the meanings given for R.sub.1 and R.sub.2; and R.sub.3', R.sub.4' and Rs' have one of the meanings given for R.sub.3, R.sub.4 and R.sub.5; are suitable as photoinitiators, especially for irradiation with light of relatively long wavelengths ##STR00001##